1. Field of the Invention
The present invention relates to the use of a coherent energy source for a high powered pulse laser, in the shock processing of materials, and more particularly to, a laser oscillator for such use, operating in a single-transverse-mode.
2. Description of the Related Art
Current lasers used in laser peening typically utilize an oscillator of the dual pump cavity type, in which the time integrated spatial profile is flat. The effect of stress birefringence in this oscillator is compensated for by a 90 degree rotator positioned between the dual pump cavities. This technique eliminates the maltese-cross pattern which is not considered desirable for laser peening.
A disadvantage of this type of dual-pump cavity oscillator, and almost any other large aperture oscillator, is that the output beam is in multi-transverse and multi-longitudinal-modes. The multi-mode nature of large aperture oscillators inherently generates hot spots in the beam because of the interaction of the different modes. These hot spots greatly decrease the lifetime of optical components through which the beam passes. Optical coatings utilized within the laser oscillator and associated laser system typically begin to show damage within ten thousand shots or bursts of the laser peening system, and typically must be replaced within one hundred thousand shots. This type of damage increases maintenance costs.
What is needed in the art is a laser peening laser utilizing a oscillator operating in a single mode to reduce damage to the associated optical components.
The present apparatus is that of a laser oscillator operating in a single-transverse mode. In one embodiment of the present invention, the oscillator is made to operate in a single-transverse mode by utilizing an iris sized to eliminate all modes but the lowest order TEMOO mode. In another embodiment a gradient reflector is used to produce a single-transverse mode. The oscillator may also include a means for compensating for stress birefringence. In addition, a telescope may be placed inside the oscillator for optimizing the diameter of the oscillator beam and for controlling thermal lensing.
In another embodiment of the invention, the oscillator utilizes a porro prism in combination with a retardation plate to reduce stress birefringence while further providing a means for generating a single-transverse-mode laser pulse.
Other embodiments of the present invention, include generating a single-transverse and single-longitudinal mode laser pulse via the addition of a seed laser or an etalon.
In one form of the invention thereof, a method for laser peening a workpiece, comprises generating a laser pulse from a substantially single-transverse mode oscillator, modifying the laser pulse with a pulse sharpening device, amplifying the laser pulse, and directing the laser pulse to the workpiece.
In another form of the invention thereof, an apparatus for laser peening a workpiece, comprises a substantially single-transverse mode laser oscillator, a laser pulse-sharpening device, means for amplifying a laser pulse, and a laser peening cell.
An advantage of the present invention is that a remarkable increase in the lifetime of components is created through the use of a single-transverse mode laser oscillator. Data confirms that such optical damage is not seen until approximately fifty thousand laser shots are utilized through the laser peening system. Without such single-transverse-mode utilization, such shots contain hot spots with energy densities greater than twice the normal level.
Another advantage of the present invention is that the lifetime of optical components may be extended at least up to five hundred thousand shots and possibly two million shots. Such extension in the component lifetime reduces maintenance costs and makes laser peening operations cost effective.
A further advantage of the present invention is that the TEMOO mode beam created has a higher density of laser energy at its center than a multimode beam and this produces a flatter beam spatial profile after amplification. An additional advantage of the present invention is that the TEMOO mode beam created has low divergence.